Macula densa cells serve in the transmission of information from tubular fluid to vascular elements through the tubuloglomerular feedback mechanism (TGF). Changes in luminal fluid [NaCl] are sufficient to activate the feedback pathway resulting in information flow from macula densa cells through the mesangial cell field and to the afferent arteriole. At the afferent arteriole, TGF signals result in alterations in membrane potential and changes in cytosolic calcium concentration ([Ca2+]i) through voltage dependent calcium channels. Our previous studies have focused on defining transport systems that operate at the macula densa since understanding these transport pathways should provide substantial insights into the TGF signaling process. With the use of microelectrodes, patch clamp techniques and fluorescence microscopy we have established the existence of apical Na:2Cl:K cotransport, Na:H exchange, K+ channel activity and a predominate CI- conductive pathway at the basolateral membrane. The purpose of these studies is to further our understanding of macula densa transport pathways and how these pathways may contribute to the generation of TGF signals using isolated perfused thick ascending limbs with attached glomeruli from rabbit kidney. The specific aims of this project are to: I) further characterize and examine the regulation of the major transport pathways located at the apical and basolateral membranes of macula densa cells; 2) determine the effects of intracellular messenger systems and physiological regulators of TGF including angiotensin Il, nitric oxide, thromboxane and adenosine on overall macula densa NaCI transport and membrane potential and 3) correlate changes in macula densa membrane potential, NaCI transport and intracellular messenger systems with TGF mediated alterations in afferent arteriole membrane potential and [Ca2+]i. These studies should provide important new insights into the generation and transmission of tubuloglomerular feedback signals from macula densa cells to arteriolar contractile elements.